This invention relates to base forming. In particular, it relates to an apparatus for forming the base of a can as it is carried on a punch of a can bodymaker.
The can bodymaker takes a formed cup and then draws and irons the cup to produce a can body. The cup is carried on a punch which passes through a series of draw and ironing rings, finally forcing the can body against a base forming apparatus. Such base forming apparatus is used for forming domed profiles on the base of a can where the base needs to be able to withstand high internal pressures, particularly for carbonated beverages. In this case, the apparatus is commonly referred to as a xe2x80x9cdoming stationxe2x80x9d or xe2x80x9cdomerxe2x80x9d.
Doming stations are traditionally high wear items which need repair or replacement much more frequently than do other bodymaker tool parts. Since the domer must be able to withstand the force of the punch carrying the can as the punch reaches the end of its stroke, failure has been commonly found in the moving parts and components which provide a reactive force to counteract the punch force. Further failure has been common in bolts which carry the domer since these bolts take any excess force during the doming cycle.
It is increasingly desirable to produce cans from lightweight materials in order to reduce material costs. However, as the materials used become thinner, so the performance of the base profile becomes more critical. The dome produced must be able to withstand not only high internal pressures but also show good drop resistance. It is thus ever more important that the tolerances of the base profiles are tight and that consistent results are obtainable from the press and, in particular from the domer.
In order to ensure that these tight tolerances are obtained consistently, it is important that the doming station is accurately aligned and that forces within the structure of the doming station are evenly distributed. Thus, the doming stations of the prior art tend to have a complicated structure to handle the loads experienced by the punch striking the dome die and require fine adjustment whenever the apparatus is stripped down for repair or maintenance.
According to the present invention, there is provided an apparatus for forming a base profile on a container mounted on a punch, the apparatus comprising a die having a face with a profile complimentary to that of the desired base profile, an adjustable hold down for clamping the container against the punch during forming of the base profile and means for biasing the die against the base of the container, arranged so that the die is moveable against the action of the biasing means as the punch reaches the end of its stroke, characterised in that the biasing means is a continuous annulus of resilient material and creates a uniform overtravel force as the punch reaches the end of its stroke.
As the hold down clamps the container against the punch, the base profile or dome is formed over the dome die. To ensure that the dome is fully formed and tolerance repeatability can be achieved, the punch must xe2x80x9cbottom outxe2x80x9d on the dome die. The die may typically be set forward of the end stroke of the punch to produce an overtravel to ensure that the punch bottoms out. The annular biasing means of the present invention provides a reaction or overtravel force as this overtravel occurs. The use of a continuous annulus of resilient material gives better force distribution than an array of independent elements which may vary in size, rating and amount of wear between individual elements. Such variations between the individual biasing elements causes uneven force distribution around the circumference of the dome die. Preferably, the biasing means comprises a polyurethane ring or a single steel spring.
Additionally, the present invention provides an apparatus for forming a base profile on a container mounted on a punch, the apparatus comprising a die having a face with a profile complimentary to that of the desired base profile and associated biasing means for biasing the die against the base of the container, and an adjustable hold down and associated biasing means for clamping the container against the punch during forming of the base profile; characterised in that the hold down biasing means is arranged surrounding the dome die.
By adapting the hold down biasing means so that it can be arranged around the dome die, in front of the support for the dome die, the apparatus of the invention is more compact than the prior art devices. Furthermore, as there is no requirement for transmission of forces from the hold down ring through or around the dome die support, transmission rods or pins are not required. This simplifies the construction of the apparatus and reduces the number of co-operating, moving parts which require bushes, bearings or seals. This in turn, reduces the number of consumable parts which are subject to wear and therefore require routine replacement.
Furthermore, as the hold down ring biasing mechanism is positioned around the dome die and there is no complex structure behind the dome die support, this arrangement facilitates easy removal of coolant from the dome die by means of a central bore which runs along the centre of the dome die and through the dome die support.
In a preferred embodiment of the invention, the hold down is enlarged to act as a piston which is biased by fluid pressure. The dome die support may then be adapted to provide the sealed piston chamber within which the hold down piston operates.
Preferably, the hold down is biased to eject the container after the base profile has been formed. The fluid pressure, which is adjustable, is typically air pressure which pushes the hold down forward after the forming operation, thereby ejecting the can from the dome tooling.
The apparatus may further comprise a sensor for detecting overtravel of the die. This sensor detects more than one thickness of material in the dome station such as when double feeds occur.
Preferably, the apparatus comprises an outer alignment ring in which the dome die, dome die support and hold down are mounted. The outer alignment ring is adjustable to ensure concentric alignment of the dome die with the punch. Once the outer alignment ring has been correctly aligned with the punch, the dome die, dome die support and hold down may be removed from the outer ring for inspection or maintenance without upsetting the alignment of the outer ring. When the dome die, dome die support and hold down are reinserted into the outer alignment ring, they self align within the outer ring. This arrangement simplifies maintenance procedures as the dome die, dome die support and hold down may be removed from the outer alignment ring and then reinserted without the need to realign the system. The outer alignment ring may be extended behind the dome die support and associated biasing means to form a housing for the dome die, dome die support and hold down.
Eccentric alignment means may be provided to adjust the orientation of the outer ring relative to the punch. These may comprise eccentric adjusters or pins which can be rotated from the rear of the dome door to align the outer ring with the punch, whilst the outer ring is located on the dome door but before the location bolts are fully tightened.
The apparatus may be adapted to be mounted in a dome door of a press by having a flanged housing, for example, which surrounds the domer and enables the domer to be locked onto the domer door. By mounting the domer in this way, cyclic loads are taken directly on the domer door rather than through bolts, thus avoiding component fatigue and risk of misalignment. None of the bolts used in the apparatus according to the invention are subjected to tensile loading during the doming operation.
The invention provides a simple and robust unit which can operate at high speed with reduced wear and simpler maintenance and alignment requirements than has been the case with prior art base forming apparatus.
Preferred embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which: